In the photolithography step of integrated circuit manufacturing, a template containing a designed set of clear and dark shapes, referred to as the mask or reticle, is repeatedly printed on the surface of a silicon wafer. This process is achieved by way of optical imaging at an image size resolution defined primarily by the wavelength (.lambda.), numerical aperture (NA) and partial coherence (.sigma.) of the optical projection system (hereinafter referred to as the stepper).
In standard industry practice as outlined in the schematic mask cross-section of FIG. 1, the masks containing the desired opaque and clear patterns are fabricated starting from an initial mask blank (FIG. 1a) consisting of a substrate which is transparent to the imaging light (10), coated on one side with an opaque film (20). Typically, the transparent substrate consists of fused silica (also known as quartz) and which will, hereinafter, be referred to as the quartz substrate whereas the transparent substrate material will be referred to as quartz. Moreover, the opaque film is typically a chromium-based material, referred to hereinafter as the chrome film, while the material of the film itself being referred to as chrome. The designed shapes are replicated on this mask blank by first selectively patterning (or "writing") the designed shapes in a protective material which is characterized as being sensitive to electron or optical exposure (FIG. 1b), hence forth referred to as the resist (30). The openings created in the resist via selective patterning (35) are then transferred to the underlying chrome film during a subsequent etch step such that, following removal of the resist material (FIG. 1c), the designed clear shapes (40) and opaque shapes (21) are replicated in the final patterned mask. Masks fabricated in this manner will be referred to, hereinafter, as standard or chrome-on-glass (COG) masks.
A different class of masks, phase-shifting masks (PSM), have demonstrated the capability of extending resolution beyond conventional imaging limits by taking advantage of both the phase and the magnitude of the imaging light. If two clear shapes which transmit light of opposite phases (180.degree. phase difference) are placed in close proximity to one another, the phase difference will produce a destructive interference null between the two shapes. Such a mask has been given several different designations in the literature such as Levenson, Levenson-Shibuya, phase edge, alternating aperture, or alternating mask. Herein, it will be referred to as an alternating mask or an alternating PSM. The additional mask fabrication steps beyond the standard mask process of FIG. 1 are shown for an etched-quartz or subtractive alternating PSM process in FIG. 2.
With reference to FIG. 2, a phase difference between two clear shapes for the alternating PSM is achieved in standard industry practice by selectively etching into the quartz substrate (10), such that an optical path difference equivalent to the desired phase offset is obtained between the two adjacent openings. Following standard mask patterning as shown in FIG. 1, a second write step is used to selectively open a protective resist coating (50) for the phase-shifted opening (41) leaving the non-phase shifted opening (42) covered, as shown in FIG. 2a. In practice, it is desirable to locate the edges of the resist pattern (55) some distance away from the phase-shifted opening (41) and on top of the opaque chrome shapes (21) where appropriate, in order to use the chrome itself as an etch barrier and to account for overlay (or pattern placement) errors between the first and second-level write steps in the fabrication process. The quartz is then etched (typically with an anisotropic reactive-ion etch (RIE) process) to a depth of approximately: EQU etch depth=phase* .lambda./[2*.pi.*(n-1)] (1)
wherein n is the refractive index of the quartz substrate at wavelength .lambda. and the phase of the opening (41) is given in radians. Following removal of the resist (50), the resultant alternating PSM has the etched-quartz trench (15) providing the desired phase difference between adjacent openings (41) and (42), as shown in FIG. 2b.
Other fabrication methods have been proposed. One of such approaches provides an accurate control of the phase, as determined by equation (1), through the addition of multi-layer films to the transparent substrate. More details may be obtained from an article by Chieu et al. entitled Fabrication of Phase Shifting Masks Employing Multi Layer Films, published in the Proc. SPIE, Vol. 2197, pp. 181-193, 1994, wherein a mask blank is described having two additional layers added between the transparent quartz substrate and the opaque chrome: an etch stop layer composed of either Al.sub.2 O.sub.3 or HfO.sub.2 and a transparent layer of silicon dioxide at a controlled thickness given by equation (1). By etching into the silicon dioxide until the etch stop layer is reached, the desired phase is then achieved. Alternatively, additive fabrication methods can be used to achieve the desired phase shift such as through the application and selective patterning of a transparent spin-on-glass following the standard mask fabrication procedures illustrated in FIG. 1. Regardless of the fabrication specifics, all of these techniques are generally categorized as an alternating PSM.
Defect-free masks are required for integrated circuit manufacturing (i.e., the patterns on the mask need to accurately replicate the designed data). In order to ensure defect-free masks following fabrication, the mask manufacturer will perform an automated optical inspection of the completed reticle to search for unwanted defects on the mask by comparing images of the mask from the optical inspection system to either the design database (hereinafter referred to as die-to-database inspection) or to the image from an exactly replicated pattern elsewhere on the mask (hereinafter referred to as die-to-die inspection). This inspection is typically performed on high-NA optical systems at wavelengths within the UV spectrum. For example, a state-of-the-art inspection system from KLA uses a 364 nm wavelength with a numerical aperture of 0.625. The defect inspection step can be further classified as either actinic (i.e., the inspection wavelength is the same as the exposure wavelength of the intended stepper) or non-actinic (i.e., the inspection wavelength is not the same as the exposure wavelength of the intended stepper).
An inspection system as described is used in standard practice for detecting defects such as shown in FIG. 3a in cross-sectional view and FIG. 3b in top-down view for the desired design shown in FIG. 3c. Opaque shapes (110) and (120) are contained on the representative mask schematics of FIG. 3a and FIG. 3b, but an extra opaque shape (130) not contained in the design data of FIG. 3c has been inadvertently added to the designed shapes as shown in FIGS. 3a and 3b. This defect is expected to cause an anomalous printing effect during lithographic patterning of the defective mask (i.e., defect 130) printing and/or causing variation in the shape or size of desired features (110) and (120) on the wafer) when the size of this opaque shape is on the order of one-third the minimum feature size or larger. Such defects may result from, but are not limited to, partial blockage of the chrome etch between the fabrication steps shown in FIGS. 1b-1c, or mask contamination by opaque, foreign material (FM) prior to mask inspection.
Generally, state-of-the-art inspection systems have demonstrated the capability to successfully find such printable defects. State-of-the-art inspection systems, however, are less adept at detecting transparent phase defects with small phase (relative to 0.degree. or 180.degree.) and/or size, and yet such defects can also have an anomalous effect on the printed image on the wafer. FIG. 4 indicates a possible phase defect on a standard COG mask comprising the small region of the quartz substrate (230) in between desired chrome shapes (210) and (220) which has been accidentally etched during fabrication or repair. The size of the transparent phase defect is determined by the lateral dimension of the defect while the phase of the defect is determined by the average depth of the defect relative to the surrounding quartz as given by equation (1). FIG. 5 provides an example of a phase defect in an alternating PSM wherein the defect consists of unetched quartz (330) within the 180.degree. designed feature (340) between opaque features (310) and (320). Watanabe et al., in the article "Detection and printability of shifter defects in phase shifting masks II., Defocus characteristics", published in the Japan Journal of Applied Physics, Vol. 31 (1992), pp. 4155-4160, Part 1, No. 12B, have demonstrated that phase defects may be more likely to print under conditions where the stepper is not operating at a focal position for which the image contrast of the desired features is maximized (i.e., the stepper is defocused from its `best` or optimum focus).
The difficulty of detecting small phase defects is demonstrated by a simulated inspection image, as shown in FIG. 6, obtained from an actinic inspection of a 20.degree. phase defect, as illustrated in FIGS. 5a and 5b, with the image shown being representative of standard industry practice today (i.e., at optimum focus with state-of-the-art optical parameters). The chrome lines (310) and (320) image at low intensity, the 180.degree. phase region images at bright intensity, while the 20.degree. phase defect (330) causes only a small anomaly as indicated.
State of the art inspection systems also have difficulty in detecting the presence and/or the integrity of phase features on the mask. FIG. 7a provides an example of a phase-shifted design in which shape (510) is assigned a 0.degree. phase, while shape (520) is assigned a phase of 180.degree.. FIG. 7b demonstrates a possible "missing shifter" configuration based on FIG. 7a, wherein pattern (530) is processed properly to match desired shape (510), even though feature (540) was not fabricated to the proper phase of 180.degree. (i.e., the phase-shift processing steps of FIGS. 2a-2b were not successfully achieved). The inspection images using standard industry practice for the properly fabricated shapes and for the missing shifter pattern are given in FIGS. 8a-8b, respectively. The small difference between the two images indicates that inspection methods practiced today are not sufficient for detecting the absence of the phase shifted pattern on shape (540).
Other techniques have been proposed to improve inspection capabilities that enable to detect the presence of phase defects. By way of example:
In U.S. Pat. No. 5,270,796 to Tokui et al., an inspection tool is described wherein a phase contrast microscope is used to generate a phase signal which is compared with a reference signal in order to detect phase defects.
Spence et al. in an article entitled "Detection of 60.degree. phase defects on alternating PSMs", SPIE vol. 3412, pp. 480-495, describe a method by which both the reflected and transmitted light from adjacent die are used in order to detect 60.degree. phase defects on alternating phase shift masks.